Soft turn-on power switching circuit

ABSTRACT

A triac is turned on during each successive half wave of AC in response to zero-crossing signals which generate a composite ramp voltage, including short sawtooth and long ramp voltages, which provides triggering signals after a predetermined threshold is exceeded. When the composite wave is continuously above the threshold, a zero-crossing switch is activated to bypass the AC signal around the soft turn-on part of the circuit, directly into the gate of the triac, to apply all of the AC power to the load after the soft turn-on has been accomplished. A circuit reset component has an input connected to the zero-crossing detector and its output connected to the generators for discharging the generators to their starting condition any time the AC power wave is interrupted, thus insuring soft turn-on even after momentary power interruptions. A regenerative pulse amplifier and a current amplifier are connected between the threshold detector and the triac for generating a triggering signal pulse for the triac and feeding back part of the threshold breaking pulse back into the threshold detector to reinforce it. The circuit can also perform a servo function by connecting an external signal to the circuit reset switch for limiting the level to which the long ramp generator ramp-shaped voltage wave can grow to below the predetermined threshold level whereby the period of operation of the triac is controlled in response to an applied external signal.

United States Patent [191 Saunders Jan.7,1975

[ SOFT TURN-ON POWER SWITCHING CIRCUIT [75] Inventor:

[73] Assignee: Display Enterprises, Inc., Rockland,

Del.

Norman B. Saunders, Weston, Mass [22] Filed: July 2, 1973 [21] Appl. No.: 375,550

Primary Examiner-A. D. Pellinen Attorney, Agent, or FirmConnolly and l-lutz [57] ABSTRACT A triac is turned on during each successive half wave of AC in response to zero-crossing signals which generate a composite ramp voltage, including short sawtooth and long ramp voltages, which provides triggering signals after a predetermined threshold is exceeded. When the composite wave is continuously above the threshold, a zero-crossing switch is activated to bypass the AC signal around the soft turn-on part of the circuit, directly into the gate of the triac, to apply all of the AC power to the load after the soft turn-on has been accomplished. A circuit reset component has an input connected to the zero-crossing detector and its output connected to the generators for discharging the generators to their starting condition any time the AC power wave is interrupted, thus insuring soft turn-on even after momentary power interruptions. A regenerative pulse amplifier and a current amplifier are connected between the threshold detector and the triac for generating a triggering signal pulse for the triac and feeding back part of the threshold breaking pulse back into the threshold detector to reinforce it. The circuit can also perform a servo function by connecting an external signal to the circuit reset switch for limiting the level to which the long ramp generator ramp-shaped voltage wave can grow to below the predetermined threshold level whereby the period of operation of the triac is controlled in response to an applied external signal.

19 Claims, 3 Drawing Figures 55 0-6 2, 1m Power S ppl! Z81 9 i 1 01 055111 Z6110 We 20 f,

Cami/10m Z i 3,859,591 SHEET 10F 3 FATENTEUJAH 71975 N 0N QNQEQQ w 0 5% mg w Q m 3 ab N Qm. uwwmw fi m fi $6 K awfi mw L fi h W NW v Nw QM wk @N NM mm 5 gang gm, z. QKWN A a A H Q REQH NWBQQW a w g 1 RW M .0 QW WM PATEHTEBJAH H975 SHEET 20F 3 ago PATENTEDJAH 71975 SHEET 3 OF 3 l is a function-of the supplied load.

after momentarypow'er interruptions; I A regenerativefl 'larly effective zero-crossing detector; includes a pair of connected. The emitters, of} the transistors are tied 'to connected to the ACphase line andltheother tied e'mli ""j m ft-iwi ema d -1{Softturn-ondevicesfare usec'l' to gradually apply'cur- "rent to sensi'tive electrical components, suchIas motors, v ;temperature FCOllttOiljCd; heaters or light vbulbs, whose 8 starting resistance is only alfr 'ac tionof normal operat- I ing resistance:Most-existing lsoft' turn-ondevices permanentlyv absorb a portion oftheop'er'ating power even.

after'a soft turn-onis accomplished and m'ayrn'ot operate after short interruptions of powerJSb me have the 'dis'advantage of utilizingonly half of the AC power wave'and b eing capable' only of performingthe soft .tur-nQOn function; An object of this invention 'is to pro vid'e'a soft turn on AC pow'erfl c'ir'c u'it which'b'ypass'es Pfull power to the load afterf-turn-on; which always oper- 'ates on turn-on even after momentarypowe'ninterrup- I tions, and which may also be utilized as a servocontrol I feedb'ack'circuit for regulating an external input which "SUMMA Y I In accordance with this invention'a thyristor, suchas a triacgfis {turned-on during each successive halfyw'ave} ofiActinresponsflrto"zemicmssmg slgnals'whl'ch generi cnhec'tedto summerf36; which may merely be a juncf I? ate acomposite r'amp voltage including short sawtooth and long'rarnp voltages,pwhich. provides triggering'sigfinals after a; predete'rni'i'edfithreshold isexceeded.

i I i i i 30.

When the cornpolsit'ej 'wave s contiiniouslyabove the threshold, a zero-crossing switch isfact'ivated to bypass the AC signal aroundthe'soft turn on=part of the circuit a reading of th efollowing description in conjunction with the accompanying drawing wherein similar refer- 1.. ence characters refer to similar parts and in which:

FIG. 1 is a schematic diagram of one embodiment of this invention; I I t FIG. 2 is-a wiring diagram for the embodiment shown in FIG. l ;.and

FIG. 3 is a diagram of the time relationships of the various waveforms at the particular points inthe circuit of. this invention. I II In FIGS. 1 and 2 is shown soft turn-oncircuit 10 for supplying a load 12,-suchfas the current sensitive light I f I, short rampgenerator flpshort rampreset26 long I v; ran1p generator 28.;Zero crossing detector l8'is also a 1 [connected tof zerocrossing switch 30 and to circuit -resetf32 to pe'rforr n'latery described functionstOu tp ut s directly into the gate of th'e triac; to apply 'all'jof the AC; n

powerato the loadafter the soft turn-on has been acconnected to the zero-crossingfldetector audits output connected t'othegenerators for' 'di schjargingthe-genera:

tors-tozth eir starting conditionany ti m e the AQ' powerl wave is interrupted, thus insuring=.softf turn-,onsevenfeeding back ,partof the thresh'old brealting:ptilse;back 1 2 into the threshold detector to reinforc'eiiti Apartictbi bipolar transistors having-tied collectors'comprijsing an output to which a; DC. power supplypotential may-b.

the bases of the othe'n-"withi onettiedemitter'andbase terand base being'refer'enced to tl ie AC commonfline. The circuit can also perforrn a-sefr vofunction byfcon;-'

necting an external signal tothe circuitjreset switch for limitingthe level to which the long tramp generator"v rampshaped, voltage wavecan grow, to. below the predetermined threshold level wherebythe;period' ofoperi ation of the thyristor is controlled in response'tolan'applied external signal. The externalfapplied signalc an'bef a function of the supplied load; such as aderived current or voltage signal, for example-from a photodetector, which modulates alight output to cause jthe'circuitj to regulate the intensity of the light. I

BRIEF DESCRIPTION OF THE DRAWING Novel features and advantages of the present invention will become apparent to one skilled inthe art from complished. A circuit-resetcomponent-has 'an'1- input 5:

I t'oI'th'e va'r'ibus components .of the-circuit I I ofthyristor,

cltiding the afo rementionedftriacorasolid state controlled,rectifirtSCRjQTriacM hasgforexamplea gate. I 62 -litliroug h wliichfsignal impulses are 's tipplied. Thy. 'risto'rs a'refa'group or class ofisolids ta tedevices that in- R and the.triac'described.'ab ove." A thy f 1 stolris a PN RN-device liavi ng1fo urlayrsI It is 'bi st'able I 'e r..all oiriorl'all'off jin response to an internal 5. -feedback"actionl A thyristor may have a signal gate," like tli'SC -R orltheltriac', or iriay befired by'eith er o f "ftwo'fg a'tes; like the'sC S tsi l'iconcontrolled switch); I {The 'triac It rQ ig'gers on ACl which canfblock. voltage, I Q f'eitherpolar ity' b'i tlwhich starts electron flow in eil I I i 4 electronpulse inor out .of-its' s1ngleLgate.-The' tr iachas fo tlr rnajor'layer s N PNP-{ F I but several N, re gion'sfare fitldiiito i t'sfllower'Playerli II "I One'of -theseNregions ,serve'sfas a terminaln'lj andthe other isithe gate;- Meanwhile} the upperiP l a'yer is made w ot e t u hpa' i fift t Nglay' -t torm iminalllafThes'e layers a ea-Jim's. y film techniques-. [Circuit '10" functionslto s'l owlyincrease; the condu I 1 .Qtionanglejofcurrent throughthe' load 12, soas to give 1; substantially constant" r'ate'l'of change of power, f'ror'nt A complete 0 to full l' OOpe rceri'tfduty'cycle and rapidly gresetupon removal of po'we rllt"also"has a simply'6 1 I l a, 'VDCpower supply consistingof-R R ;D1, D; and. C, g

whose'oiitput is regulated'by the LIMITER; Q an ava' .j lanche diode. r P

bulb off' a film projectonjthrough thyristor l4, s uch as a triac,;from a 115' volt A C supply including AC phase -line l6'andAC-common line 17. The period ofoperation o f triac l ll-iscontrolled during each half wave of. QAC power in response tothe zero-crossing portions of vthe AC power wave ,vvhfich are detected by ,zero- 6 "crossing detector.'fl8 having:an input 20 connected to the'A C phaSe'lineIG and 'anoutput 22 connected to 33 and from thef shortand long ramp generators are tion; land which isfin"turncon'nected through" output line '38 -tol- "th'reshold detector-.40: 'Output. 42 from ..threshold detector40 i'sco'nnected through input 4410 1 '-'regenerative pulsearri-plifi'er'46.0utput SQof regenera tive'pul'se; amplifier 46 is connected ;througl'1 inpu't-; 5 2' to current amplifier 54 and throughlinput 481d i'ero crossing switch-j BOQPiart of output:'JSO J is connected throughfeedjback lineS6 back to summer-36';tofrein "y force'.the-initiallyweakthresholdbrakingsignaLI D'C :1: g ',power sup lygs s is connected thrlough 1 li m'ite n' '6( t o= 'pr ovide alDCreferencepotentialyfor egtample"o'volts i Thyrisltor 14 isatiy'signal triggereditype;

ciiide'thel'S and Zero-crossing detector 18 uniquely includes transistors Q and Q connected as shown in FIG. 2. The collectors of transistors Q and Q, are tied together to provide a common output. The emitters of Q and Q are tied to the bases of the other. The base of Q and emitter of are tied to the AC phase line 16 through R The tied emitter of transistor Q and base of 0 are connected to the AC common line 17 through line 64. The tied collectors of Q and 0 provide an output signal each time the AC power wave crosses zero as later described.

Transistors Q and Q sense the zero-crossings of the line voltage through resistor R every half-cycle after power is applied, by remaining off when the line'voltage, is within a narrow zone near zero, and by being on all the rest of the time during each half-cycle: Q for positive half-cycles, 0 for negative half-cycles.

When line voltage is in the zero-crossing zone, 0., is biased on, setting the potential across C to zero volts. During this brief period, base current flowing through R, applies a small amount of charge on C Additional small amounts of charge are placed on C at each successive zero-crossing. The major charge comes from continuous current through R to C slowly raising the voltage across that component (long ramp generator). Toward the end of the long ramp more andmore of this current is diverted to the base of Q which occurs from time of firing to the next zero-crossing.

When line voltage reaches a value to turn on Q or Q3, O is turned off allowing the current flowing through R, to charge C the short ramp generator. The voltage at the base of transistor Q, is the sum of the voltages appearing across C and C After a sufficient number of half-cycles have occurred, (approximately 15 with the values chosen), the voltage at the base of Q reaches a level sufficiently high to-turn on Q late in the half-cycle. With the continually increasing voltage across C and the recharging of C, during each half-cycle, this point is reached earlier with each successive half-cycle of the AC line until the voltage across C, is itself great enough to bias Q, on after which time additional excursions of the line voltage have no effect, and the soft turn on phase is completed,

During the soft turn on phase, each time the 0 turn on threshold is reached, a regenerative reaction occurs due to the switching action of transistor Q causing the collector currents through R to raise the voltages on C and C thereby pushing more current into the base of Q Transistors Q and Q, thus form a 2-stage amplifier of the regenerative type (univibrator) whose timing (pulse length) is controlled by C and R uSec). The threshold is set by the potential divider R R and the base-emitter potential of Q Transistor Q also performs as an emitter follower or current amplifier turning on 0,, through C Transistors Q Q and 0 together constitute a multi-stage current.

amplifier, transforming the 10 micro-ampere pulse current at the base of 0 into a 200 milliampere pulse at the collector of Q,,. R serves to limit this current. R establishes the voltage necessary to turn on the Triac AC line switch in series with the line load. C, eliminates spurious turn-ons resulting from dV/dt effect within the triac switch when AC power is first applied to the circuits.

During the soft turn on O is inoperative. But, after the soft turn on phase is completed and transistor 0,, is

held on continuously, the emitter of Q is pulled substantially toward the base of Q decreasing 0 baseemitter voltage, permitting O to be turned on at each zero-crossing, and thereby switching on the triac for the full half-cycle each half-cycle thereafter. Hence, Q serves to bypas the soft turn on circuitry after it is not longer needed. The diode D puts the low frequency components, and the capacitor C puts the transients of the zero-crossing pulse that exist atthe collectors of Q and'Q into the base of Q (during the soft turn on phase) to turn it on and Q and Q off before this voltage at the base of Q risesfar enough to turn, Q, on. This presents 0 from being turnedon in response to spurious signals. The resistor R limits the current through Q when O is turned on.

' During both soft turn-on and normal operating phases of the device, transistor Q is held off by the average collector potential of Q and 0 being so low as to never permit large charge accumulation on C When power is removed from the circuits, charge stored at C is permitted to transfer to C bias 0., and O to the on state, discharging C and C and thereby quickly resetting the ramp generators for the next application of line power.

External connections X and Y or Y and Z provide inputs for using circuit 10 as a modulator or controller. A function of load 12, such as light output, can be maintained constant by applying the monitoring signal to external input terminals or connections X, Y and Z in various combinations. The externalsignal applied through R-, is applied to circuit resetswitch O to limit the level to which the long ramp generator rampshaped voltage wave can grow to below the predetermined threshold level. This controls the period of operation of triac 14 in response to the applied external signal to increase or decrease the current applied to the load in order to maintain it constant. Circuit 10 can thus provide soft turn-on and thereafter perform a modulating or monitoring servocontrol feedback function to control a load responsive function. The regulating signal can be derived from a photo detector such as a photo diode or a'photo resistor or from any current or voltage-producing transducer to which the load output is applied, or any external signal source.

Soft turn-on circuit 10 has various advantages including a positive delay of power application which slowly edges into full power, thus allowing other switch loads to settle before full power is applied. The extremely gradual voltage increase may allow brighter lighting bulbs to be utilized because full voltage is not applied until their filaments become fully heated and develop full resistance. The thyristor, such as a triac or solid state control rectifier, is continually turned on after soft turn-on exactly at zero-crossings, of the AC line, which minimizes power loss and radio frequency interference. The circuit reset insures soft turn-on even after momentary interruptions of power which fully protects the output load and provides rapid automatic reset after interruptions. As previously mentioned, the circuit can also be utilized for automatic voltage, current or intensity control, even in response to low power input signals. The circuit is inherently insensitive. to the switching transients. It also is simple and economical by virtue of utilizing several components for a multitude of functions. Most other soft turn-on circuits have either a variable resistance in series with the load, which is never completely removed or an inherent delay in turn on after zero-crossing. This prevents full utilization of applied power, which this present circuit advantageously provides.

In FIG. 3 are shown the time relationships of the principal waveforms occurring in indicated portions of the circuit of this invention. Each of these curves are designated by letters A-J corresponding to points in the wiring diagram, FIG. 2, upon which these letters are also applied. The components at which these waveforms occur are also designated on FIG. 3.

Waveform A shows the standard AC power sine wave with which each of the following waveforms are coordinated.

Waveform B shows the zero-crossing signal provided at the base of transistor Q and the emitter of transistor Q which constitute the zero-crossing detector. A steep vertical signal is provided each time the AC power line crosses the zero axis. Waveform C is the signal derived at the tied collectors of zero-crossing detecting transistors and Q showing the sharp spike shaped signal at each zero-crossing. Waveform D shows the sharp signal occurring at the base of transistor 0 at zero-crossing which governs the discharging of short ramp generator C, at the end of each half-cycle as the incremental additional charging of the long ramp generator C Waveform E shows the composite waveform at the junction of the collector of Q and the base of Q which is limited to the threshold level set by R R and the base emitter voltage of Q Waveforms F and G are the short ramp generator C voltage waveform and the long ramp generator C voltage waveform, which are combined to provide the composite waveform E.

Waveforms H and C respectively show the signals at the tied emitters of Q and Q (H) and the voltage at the base of Q (C). These waveforms combine to turn bypass transistor 0 fully on after the soft turn on phase is completed.

Final waveform J shows the signal at the junction of R and R and the triac gate indicating how the triac is turned on earlier during each successive half cycle to gradually provide more and more AC power during each half-cycle to the load until it is continuously on. Waveforms E-.! are compressed and the slope of the curves and attainment of threshold bypass conditions are indicated earlier than actual to facilitate illustration in the limited drawing space. With the following advantageous set of component values, the triac will be turned on the first time late in the half-cycle approximately cycles after AC power is applied to the circuit, and will not be turned on at the beginning of each half-cycle until approximately 800 more cycles have occurred to thus end the soft turn on phase.

The following is an advantageous set of values for the resistors and capacitors shown in FIG. 2 of this application.

52% R1 47 R R2 47 k R3 47 k R4 56 k R5 470 k R6 560 k R7 1 k R8 220 ohms R9 680 k -Continued Resistors R10 220 ohms R11 33 k R12 10 k R13 10 k R14 k R15 10 k R16 1 k R17 33 ohms R18 1 k C 1 6O [.Lf, 10 v C 2 47 pf C 3 47 nt C 4 47 nf C 5 Z 2 pf C 6 10 nf C 7 approx. 0.2 pf

I claim:

1. A power switching circuit for controlling the application of an AC power source to a load through a thyristor turned on by actuating signals applied to its gate comprising a zero-crossing detector having an input connected to said AC power source for generating a zero-crossing signal each time the AC wave passes through zero, a short ramp generator and a short ramp reset for generating a sawtooth voltage wave in response to each zero-crossing signal, a long ramp generator for generating a ramp-shaped voltage wave which grows in response to each successive zero-crossing signal, the outputs of said generators being connected to a summer which adds them into a composite ramp wave, a threshold detector connected to said summer for determining when said composite wave rises above predetermined threshold level whereby a turn-on signal is generated, said threshold detector being connected to said gate for turning on said thyristor at earlier times during each successive AC half-cycle wave, a zerocrossing switch connected between said zero-crossing detector and said gate for applying said zero-crossing signal directly to said gate, said threshold detector being connected to said zero-crossing switch for enabling it to be turned on after said composite ramp wave is completely above said predetermined threshold level whereby said zero-crossing signal is bypassed from said short ramp generator and said short ramp reset and said long ramp generator and directed through said zero-crossing switch into said gate for causing said thyristor to apply all of said AC power to said load.

2. A circuit as set forth in claim 1 wherein a circuit reset means has an input connected to said zerocrossing detector for making said circuit reset means inoperative when said AC power wave is being provided, said circuit reset means having an output connected to said short and long ramp generators for dis charging them to their initial bottom condition when said AC power is off whereby said circuit is reset to bottom starting level upon interruption of power.

3. A circuit as set forth in claim 2 wherein an external connection means is provided to said circuit reset means for connecting an external signal which limits the level to which said long ramp generator rampshaped voltage wave can grow to below said predetermined threshold level whereby the period of operation of said thyristor is controlled in response to said applied external signal.

4. A circuit as set forth in claim 3 wherein said external signal is a function of the power applied through said thyristor to said load whereby said circuit is constituted to perform a servo control function.

5. A circuit as set forth in claim 1 wherein a pulse amplifier is connected between said threshold detector and said gate for providing signal pulses to turn said thyristor on when said composite ramp wave is above said threshold level.

6. A circuit as set forth in claim 5 wherein said pulse amplifier and threshold detector act together in a regenerative manner.

7. A circuit as set forth in claim 5 wherein a current amplifier is connected between said pulse amplifier and said gate for amplifying said signals.

8. A circuit as set forth in claim 1 wherein a current amplifier is connected between both said threshold detector and said zero-crossing switch and said gate whereby signals from said threshold detector and said zero-crossing switch to said gate are amplified.

9. A circuit as set forth in claim 8 wherein said current amplifier includes input and bias circuitry, said zero-crossing switch having a collector load, and common components in said current amplifier and said zero-crossing switch comprising both said input and bias circuitry and said collector load.

10. A circuit as set forth in claim 9 wherein said common components comprise series-connected capacitor and resistor means.

11. A circuit as set forth in claim 1 wherein a pulse amplifier is incorporated within said threshold detector and said gate, and connection means connecting part of the output of said pulse amplifier back into said threshold detector whereby the strength of the output of said threshold detector to said gate is augmented.

12. A circuit as set forth in claim 11 wherein a current amplifier is connected between said pulse amplifier and said gate said current amplifier having input and bias circuitry, said pulse amplifier having an emitter-follower load, and common components in said current amplifier and said pulse amplifier comprising both said input and bias circuitry and said emitter-following load.

13. A circuit as set forth in claim 12 wherein said common components comprise series-connected capacitor and resistor means.

14. A circuit as set forth in claim 1 wherein said zerocrossing detector comprises a pair of bipolar transistors having collectors tied.together, the emitters of each transistor being tied to the base of the other, said AC power source having a phase and a common line, the base of one transistor and emitter of the other being tied to said AC phase line, and the emitter of said one transistor and the base of said other transistor being referenced to said AC common line, said transistors being constructed and arranged to turn off and on each time said AC wave passes through zero whereby said zero-crossing signals are provided at the output of said transistors comprised by said tied collectors.

15. A circuit as set forth in claim 14 wherein a DC power supply potential is connected to said tied collectors.

" 16. A circuit as set forth in claim 14, wherein said zero-crossing detector provides an output'signal between said tied collectors and said AC common line whenever the voltage at the connection to said AC phase line is smaller than the threshold voltage of either transistor, one transistor sensing the AC phase line voltage condition during its positive swing, the other transistor sensing said AC phase line voltage condition during its negative swing, both said transistors acting in concert to provide a voltage pulse at said output when said AC phase line is in a narrow zone at zero-crossing, said output voltage pulse starting just prior to, ending shortly after, and spanning the instant of each said AC phase line voltage zero-crossing.

17. A circuit as set forth in claim 1 wherein said thyristor comprises a triac.

18. A circuit as set forth in claim 1 wherein a DC power supply potential is connected to said zerocrossing detector, short ramp reset, short ramp generator, long ramp generator, threshold detector, zerocrossing switch, pulse amplifier, current amplifier and circuit reset means.

19. A circuit as set forth in claim 1 wherein said short and long ramp generators comprise capacitor means and a charging circuit which they share in common. 

1. A power switching circuit for controlling the application of an AC power source to a load through a thyristor turned on by actuating signals applied to its gate comprising a zero-crossing detector having an input connected to said AC power source for generating a zero-crossing signal each time the AC wave passes through zero, a short ramp generator and a short ramp reset for generating a sawtooth voltage wave in response to each zerocrossing signal, a long ramp generator for generating a rampshaped voltage wave which grows in response to each successive zero-crossing signal, the outputs of said generators being connected to a summer which adds them into a composite ramp wave, a threshold detector connected to said summer for determining when said composite wave rises above predetermined threshold level whereby a turn-on signal is generated, said threshold detector being connected to said gate for turning on said thyristor at earlier times during each successive AC half-cycle wave, a zero-crossing switch connected between said zero-crossing detector and said gate for applying said zero-crossing signal directly to said gate, said threshold detector being connected to said zero-crossing switch for enabling it to be turned on after said composite ramp wave is completely above said predetermined threshold level whereby said zero-crossing signal is bypassed from said short ramp generator and said short ramp reset and said long ramp generator and directed through said zero-crossing switch into said gate for causing said thyristor to apply all of said AC power to said load.
 2. A circuit as set forth in claim 1 wherein a circuit reset means has an input connected to said zero-crossing detector for making said circuit reset means inoperative when said AC power wave is being provided, said circuit reset means having an output connected to said short and long ramp generators for discharging them to their initial bottom condition when said AC power is off whereby said circuit is reset to bottom starting level upon interruption of power.
 3. A circuit as set forth in claim 2 wherein an external connection means is provided to said circuit reset means for connecting an external signal which limits the level to which said long ramp generator ramp-shaped voltage wave can grow to below said predetermined threshold level whereby the period of operation of said thyristor is controlled in response to said applied external signal.
 4. A circuit as set forth in claim 3 wherein said external signal is a function of the power applied through said thyristor to said load whereby said circuit is constituted to perform a servo control function.
 5. A circuit as set forth in claim 1 wherein a pulse amplifier is connected between said threshold detector and said gate for providing signal pulses to turn said thyristor on when said composite ramp wave is above said threshold level.
 6. A circuit as set forth in claim 5 wherein said pulse amplifier and threshold detector act together in a regenerative manner.
 7. A circuit as set forth in claim 5 wherein a current amplifier is connected between said pulse amplifier and said gate for amplifying said signals.
 8. A circuit as set forth in claim 1 wherein a current amplifier is connected between both said threshold detector and said zero-crossing switch and said gate whereby signals from said threshold detector and said zero-crossing switch to said gate are amplified.
 9. A circuit as set forth in claim 8 wherein said current amplifier includes input and bias circuitry, said zero-crossing switch having a collector loAd, and common components in said current amplifier and said zero-crossing switch comprising both said input and bias circuitry and said collector load.
 10. A circuit as set forth in claim 9 wherein said common components comprise series-connected capacitor and resistor means.
 11. A circuit as set forth in claim 1 wherein a pulse amplifier is incorporated within said threshold detector and said gate, and connection means connecting part of the output of said pulse amplifier back into said threshold detector whereby the strength of the output of said threshold detector to said gate is augmented.
 12. A circuit as set forth in claim 11 wherein a current amplifier is connected between said pulse amplifier and said gate said current amplifier having input and bias circuitry, said pulse amplifier having an emitter-follower load, and common components in said current amplifier and said pulse amplifier comprising both said input and bias circuitry and said emitter-following load.
 13. A circuit as set forth in claim 12 wherein said common components comprise series-connected capacitor and resistor means.
 14. A circuit as set forth in claim 1 wherein said zero-crossing detector comprises a pair of bipolar transistors having collectors tied together, the emitters of each transistor being tied to the base of the other, said AC power source having a phase and a common line, the base of one transistor and emitter of the other being tied to said AC phase line, and the emitter of said one transistor and the base of said other transistor being referenced to said AC common line, said transistors being constructed and arranged to turn off and on each time said AC wave passes through zero whereby said zero-crossing signals are provided at the output of said transistors comprised by said tied collectors.
 15. A circuit as set forth in claim 14 wherein a DC power supply potential is connected to said tied collectors.
 16. A circuit as set forth in claim 14, wherein said zero-crossing detector provides an output signal between said tied collectors and said AC common line whenever the voltage at the connection to said AC phase line is smaller than the threshold voltage of either transistor, one transistor sensing the AC phase line voltage condition during its positive swing, the other transistor sensing said AC phase line voltage condition during its negative swing, both said transistors acting in concert to provide a voltage pulse at said output when said AC phase line is in a narrow zone at zero-crossing, said output voltage pulse starting just prior to, ending shortly after, and spanning the instant of each said AC phase line voltage zero-crossing.
 17. A circuit as set forth in claim 1 wherein said thyristor comprises a triac.
 18. A circuit as set forth in claim 1 wherein a DC power supply potential is connected to said zero-crossing detector, short ramp reset, short ramp generator, long ramp generator, threshold detector, zero-crossing switch, pulse amplifier, current amplifier and circuit reset means.
 19. A circuit as set forth in claim 1 wherein said short and long ramp generators comprise capacitor means and a charging circuit which they share in common. 